Internet sources indicate that most NAC is manufactured in China using cysteine from animal protein, mostly through the hydrolysis of human hair or poultry feathers. I contacted Metabolic Maintenance to inquire about the manufacturing source of their NAC. While they would not identify the exact manufacturer, they did say that it was manufactured in Japan by synthesis (ie. not from human hair or feathers). This may explain why NAC of other brands costs less than that from Metabolic Maintenance. Because of this and because Metabolic Maintenance does not use any fillers in their NAC product, I prefer this brand for NAC.

I bought this based on the first review because I had not found an NAC I could tolerate and this one did not have anything in it that is problematic for me. Yes, it's more expensive than the others, but I am glad I got it, as it's the only one that I seem to tolerate out of all the ones I have tried. A big thank you to the person who wrote the other review and said what the difference was with this product. My doctor wanted me to take 600 mg once a day, and now I'm glad that I can!

I have suffered with chronic fatigue and shortness of breath for over twenty years I take two in the morning and one in the afternoon can't believe what a difference it has made I can now get out of bed and actually accomplish things so glad I am using it